Target element for electrostatic storage display tube



J. M. ENGEL Feb. 24, 1970 TARGET ELEMENT FOR ELECTROSTATIC STORAGE DISPLAY TUBE Filed Jan. 28, 1969 FlG.-I

SPUTTERED SILICON DIOXIDE LAYER 50 (SPUTTIERED SILICON DIOXIDE LAYER MICA TARGET 26 INVENTOR F I G 2 JAN M. ENGEL ATTORNEYS United States Patent Int. Cl. H013 29/00 US. Cl. 31389 2 Claims ABSTRACT OF THE DISCLOSURE In an electrostatic storage display tube, fogging of the target element by toner particles is eliminated by a surface in contact with the particles that is triboelectrically compatible with the toner particles. Such a surface may be provided by sputtered silicon dioxide.

CROSS-REFERENCE TO RELATED APPLICATION This application is a continuation-in-part of application Ser. No. 608,881, I an M. Engel, Target Element for Electrostatic Storage Display Tube, filed Jan. 12, 1967, and assigned to a common assignee.

BACKGROUND OF THE INVENTION Field of the invention The present invention pertains to electrostatic storage display tubes and more particularly to an improved target element for tubes which form a desired pattern by electrostatically attracting particles of toner material to the target element.

Description of the prior art Electrostatic storage display tubes as known today developed from experimentation with cathode ray tubes. 1*. was found that if the fluorescent screen within a cathode ray tube was replaced by a non-conductive or-dielectric material, the surface of which was exposed to the electron beam, a charge pattern was produced upon the dielectric material. A permanent image of the information could then be provided by depositing certain powders on the dielectric material. In early tubes, such as that disclosed in US. Patent No. 1,818,760 of Selenyi, it was found that the glass walls of most tubes provided a target surface on which the particle deposit could be made either from the inside or the outside of the tube. The inconvenience of manually removing the particles from the target surface in preparation for a new image led to the development of tubes wherein the target surface could be dusted and erased much more easily to facilitate the more rapid printing of successive diiferent images. At the same time, glass targets were abandoned in favor of target materials having the thinness needed for better resolution and contrast, and the ability to withstand severe pressure differentials present during the formation of tubes having separate development and electron gun chambers.

While ideally the charges imparted to the dielectric target are confined to certain desired areas of the target to provide a high resolution, high contrast pattern, as a practical matter many of the target materials now used eventually become charged at least to some degree. With repeated operation, the particles are attracted at random to most or all areas of the target surface, and cloud or fog the visual image. Coating the target surface with different materials such as an evaporated film of silicon monoxide has been attempted, without particular success in reducing or eliminating the fogging problem.

3,497,748 Patented Feb. 24, 1970 "ice In accordance with the present invention, clouding or fogging of the visual image is prevented by utilizing a material that is generally triboelectrically compatible with the toner particles for the surface of the target element of an electrostatic display tube that contacts the toner particles. Certain oxides of silicon having a greater degree of oxidation than silicon monoxide, such as silicon dioxide and Si O have been found to provide the desired compatibility with some toner particles of generally organic composition or of the electrographic type. When deposited in a layer on an existing target screen of material such as mica which is generally triboelectrically incompatible with most toner materials, the oxides present a surface in contact with the particles which does not become charged in a manner to cause fogging, and which does not interfere with the electrostatic system. Silicon dioxide to be used as the target surface is preferably sputtered onto the target element in an atmosphere ofinert gas and oxygen, thereby avoiding an undesired equilibrium reaction which may lead to reduction of the silicon dioxide. It has been found that a layer of silicon dioxide sputtered to a thickness of approximately 1200 A. is satisfactory for most applications.

BRIEF DESCRIPTION OF THE DRAWING Objects and advantages other than those indicated above will be apparent from the following description when read in connection with the accompanying drawings, in which:

FIG. 1 is a perspective view, partly in section, illustrating an electrostatic storage display tube having an improved target element in accordance with the present invention; and

FIG. 2 is a sectional enlarged view of the improved target element of the tube of FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENT FIG. 1 illustrates one particular arrangement of an electrostatic storage display tube including an improved target element in accordance with the present invention. It should be understood, however, that the particular tube arrangement of FIG. 1 is provided by way of example only, and the improved target element can be used with any electrostatic display tube configuration.

The display tube of FIG. 1 includes an evacuated envelope 10 of a suitable material, such as glass or metal, and of any suitable shape, such as that of a conventional cathode ray tube having an electron gun and focusing system at one end and an enlarged face plate or screen member at the other end. In the system of FIG. 1, individual write and flood guns are disposed in separate necks extending from the envelope 10. The write gun 12 includes a cathode 14 and a control grid 16 to which signals are applied to modulate the electron beam intensity in the conventional fashion. The modulating input signals may be derived from any suitable source, such as from video information signal circuits 18. The modulated electron beam is scanned in raster fashion across the target surface by suitable deflection means, such as a deflection yoke 20 disposed around the neck of the envelope 10, and controlled by a sweep voltage generator 22. 'Although a video-type scanning system is shown, the display may also be generated by digital-type circuits or by char acter beam-type tubes as desired for a particular application.

A transparent face plate 24 is provided at the end of the envelope 10 opposite the write gun 12, and may either be a unitary part of the envelope 10 or, as shown, a separate air-tight element. Spaced from, but adjacent the transparent face plate 24, is an electrical insulating member or target screen 26 on which an electrostatic charge pattern is created by the electron beam. The target screen 26 is firmly supported in the evacuated envelope 10 such as by a split clamping ring 30 seated against a shoulder in the end of the envelope 10.

The material comprising the target screen 26 must be carefully chosen to provide various desirable properties important to the successful construction and operation of the display tube. Such material must be capable of fabrication in a relatively thin sheet, preferably on the order of 10 microns or less in thickness, to achieve acceptable or superior resolution of data to be displayed. Targets which are much thicker than this result in a substantial spreading of the electrostatic fields at the target Surface adjacent the development chamber providing generally unacceptable resolution. At the same time, however, the resulting thin sheet comprising the target screen must be capable of withstanding bake-out temperatures on the order of 400 C. during manufacture of the tube. The target screen must also be capable of withstanding substantial differences in pressure, both during manufacture of the display tube and during subsequent use thereof, particularly where the tube is to be of the sealed type rather than the demountable type. In a display tube of the sealed type, the separate electron gun and developing chambers are commonly reduced to pressures on the order of 10' 10- torr. and 10- 10 torr. respectively. During manufacture of the tube, the pressure differential is normal ly much greater than this due to the independent evacuation of the separate chambers. Resolution of the display tube is dependent on the material of the target screen having a very high resistivity, preferably on the order of ohm-cm. or greater, as well as being very thin. Optical factors may further dictate that the target screen be highly transparent where transmissive optics are used, or that the screen present surfaces which are very flat and smooth where reflective optics are used.

Materials such as glass are not suitable for use as the target screen 26, primarily because they cannot be made in the small thicknesses on the order of 10 microns or less required for acceptable contrast and resolution. Moreover, even if it were possible to produce a glass target in the required thinness, the resulting target would not be able to support itself even in the absence of the pressure differentials which are typically present within the tube.

Materials which have been found to be satisfactory for use as the target screen 26 include mica, polyester resin film sold under the designation Mylar, and polyvinyl fluoride sold under the designation Tedlar. Such materials can be made in a desired thickness and have acceptable resistivities on the order of 1O "l0 ohm-cm. at room temperature. Mylar is preferred for certain applications in which transmissive optics are used because of its very high resistivity and transparency. Tedlar is generally opaque, and is therefore useful with reflective optics. The resistivity and transparency of mica, however, are satsifactory for most tube applications employing transmissive optics, and mica is accordingly preferred over the other materials for use in sealed tubes because of its strength and high melting point which is typically on the order of 1500 C.

A separate developing chamber 32 is defined by the volume between the transparent target screen 26 and the transparent face plate 24. Within the developing chamber 32 is maintained a loose mass of particulate matter or developer, apart of which is capable of retaining an electrostatic charge. The developer includes a pigment consisting of toner particles 36, and a carrier comprising dissimilar particles 38. The toner particles 36 must be capable of assuming a net positive charge and of retaining the change to effect development of an image through attraction to negatively charged areas of the target screen 26. The particles 36 should be relatively dark in color to provide adequate contrast where reflected light is used, and are preferably opaque where transmitted light is to be used, as hereafter described. The material of the toner particles must be chemically stable so as not to react with other materials of the developing chamber 32, and is preferably although not necessarily a dielectric in order to aid in charge retention. Such material which is typically a synthetic resin or plastic may be organic or inorganic as appropriate, and may be of the electrographic type where desired. The carrier 38 comprises any appropriate particles such as suitably treated glass beads or suitably treated flint shot which will triboelectrically produce a net positive charge on the toner particles 36 when agitated therewith and which will carry the charged toner particles 36 across the target screen 26 for attraction to the charged areas thereof.

The toner particles 36 and the dissimilar particles 38 are maintained physically separate from the target screen 26 and the electrostatic charge distribution pattern developed thereon during the writing process, being held in the bottom of the envelope 10 within the developing chamber 32. Upon completion of the electrostatic writing, means are employed to triboelectrically charge the toner particles 36 through agitation with the dissimilar particles 38. In FIG. 1, the end of the envelope 10 adjacent the developing chamber 32 is made rotatable relative to the target screen 26 and the remainder of the tube. A motor 33 drives the end portion of the envelope 10 through a gear train 34 or other suitable drive. When the writing is completed, the tube end is rotated by energizing the motor 33, thereby charging the toner particles 36 and cascading them across the target screen 26 in desired fashion. Other arrangements for cascading are equally suitable such as, for example, those whereby the entire display tube is pivotable about a horizontal axis, the tube being maintained in a substantially horizontal position except during cascading when the tube is pivoted into a substantially vertical position to cascade the charged particles across the target screen 26.

A flood gun 40 operated by erase control circuits 42 is utilized for erasure of the electrostatic charge distribution pattern on the writing side of the target screen 26. As shown in FIG. 1, the flood gun 40 is mounted in a Separate neck of the envelope 10 and directs a dispersed high denisty beam toward the target surface 26. Details of the beam focusng and accelerating system have not been shown because they may be conventional. The flood gun 40 is operated at sufficient energy levels to provide the accelerating voltages needed to establish a secondary emission ratio greater than unity at the target screen 26. As is well known, the secondary emission ratio of many insulating materials becomes greater than unity when bombarded by an electron beam having an accelerating potential in a given range. Within this accelerating potential range, the ratio of electrons emitted by the target screen 26 to those striking the target screen 26 exceeds unity. Under the flood beam bombardment, the electrons forming the negative electrostaic charge on the target screen 26 are removed, dissipating the electrostatic charge and releasing the adhering electrostatically charged toner particles 36. During erasure, the loose masses of particles 36 and 38 are again cascaded across the reverse side of the target screen 26 to remove charged particles that still adhere.

The electrostatic display tube of FIG. 1 is arranged so that the electrostatic charge pattern may be projected upon the viewing screen 28. A high intensity light beam from a projection lamp 44 is passed through condensing lenses 46 and across the central axis of the tube 10 through the target screen 26 and an exit window 47 to a projection lens 48 from which the image is projected onto the viewing screen 28. A visible image of virtually any desired size may therefore be produced, the size being limited only by the quality of the optical components, the amount of light available from the light source, and the space available. It may be seen that this system comprises a form of light valve, in which low energy levels in the form of electrostatic charges are used to modulate, or provide a spatial pattern within a high intensity beam. Other physical dispositions of the principal operating .units may be employed, as well as completely different electrostatic display tube systems, and the arrangement of FIG. 1 is provided by way of example only.

As previously mentioned, materials such as Mylar, Tedlar and mica have been found to work well as the target screen 26 in an electrostatic storage display tube, with mica being preferred for use in the sealed tube because of its strength and stability at temperatures customarily used for processing vacuum tubes. Mica target screens have a serious limitation, however, in that when used with most toner particles of organic composition the target surface eventually clouds or fogs, blurring the image and seriously affecting the usefulness of the tube. Coating the target surface with different materials such as an evaporated film of silicon monoxide has been attempted, without particular success in reducing or eliminating the fogging problem.

In accordance with the invention, it has been found that the clouding or fogging of the target surface stems from the general triboelectric incompatibility between certain target screen materials such as mica and most of the more commonly used toner particle materials. The triboelectrification or frictional electrification of various materials which undergo mutual agitation or frictional contact can be observed and compiled in the form of a triboelectric series in which the higher materials are charged positively relative to the lower materials and the charge manitude is generally dependent upon the extent to which two different materials are separated from one another within the series. Examination of a triboelectric series containing mica as well as various ones of the materials commonly used as the toner particles indicates that mica is below and considerably removed from most such materials. As the toner particles are cascaded across the surface of the mica target screen during repeated use of the display tube, the frictional contact therebetween results in the eventual buildup on the target surface of a charge pattern which is random and generally independent of the charge patterns produced by the electron beam. The high resistivity of the mica prevent dissipation of this unwanted surface charge, and the charge attracts toner particles producing the clouding or fogging effect noted, particularly where the mica lies below the toner particle material within the triboelectric series so as to charge the target surface negatively relative to the positively charged particles.

In accordance with the invention, the clouding or fogging which results from the use of toner particles and target surfaces which are generally triboelectrically incompatible is eliminated by a thin surface layer 50 of material triboelectrically closely related or comaptible with the toner particle material and which is deposited on the surface of the target screen 26 adjacent the developing chamber 32 as shown in FIG. 2. The material of the surface layer 50 is chosen from the triboelectric series so as to be relatively close to the material of the toner particles, and accordingly charging of the surface thereof through repeated frictional contact with the toner particles is negligible. The position of the surface layer material within the series can be either above or below the toner particle material, but is preferably above that of the particle material so that any charge on the surface layer 50 which may result is positive and therefore tends to repel the positively charged toner particles, but not so positive that the charged toner particles are repelled even when a negative charge has been placed on the other side of the target screen 26 by the write gun electron beam. In addition to its triboelectric properties, the material of the surface layer 50 preferably has a relatively high transparency, a relatively high resistivity to prevent removal of charge from the toner particles, a relatively smooth and fiat surface to prevent entrapment of the toner particles and a melting point which will enable it to withstand the bake-out temperature during the manufacture of a sealed tube.

Materials which have been found to work well as the surface layer 50 when used with a mica target screen 26 and most of the commonly used toner particle materials includes oxides of silicon such as silicon dioxide (SiO and Si O having a greater degree of oxidation than silicon monoxide. Such materials moreover do not attenutae the electrostatic forces of attraction or effect the light transmissivity of the target screen. The resistivities of such materials are such as to prevent the dissipation of the charges on the toner particles, the resistivity of silicon dioxide for example 'being on the order of 10 "-1() ohmcm. at room temperature. Such materials present a surface which is smooth and uniform and which does not tend to entrap the toner particles.

In a preferred electrostatic storage display tube constructed and tested in accordance with the invention, a layer of silicon dioxide was sputtered to a thickness of approximately 1200 Angstrom units on the surface of a mica target screen. Silicon dioxide and Si O can be added to the target screen surface by methods other than sputtering, such as evaporation. The sputtering process, however, is the best one thus far found to provide satisfactory results in terms of control of thickness, uniformity and operative characteristics. The sputtering is preferably carried out in an atmosphere of inert gas and oxygen, the oxygen present acting to prevent an equilibrium reaction which leads to undesirable reduction of the silicon dioxide. The deposited layer may be in amorphous or crystalline form. As to other materials that are triboelectrically compatible with the toner particles, it should be noted that some are unstable under typical operating conditions or are not amenable to disposition in the needed fashion.

While the invention has been particularly shown and described with reference to a preferred embodiment thereof, it will be understood by those skilled in the art that various changes in the form and details may be made therein without departing from the spirit and scope of the invention.

What is claimed is:

1. Apparatus for forming a visible image from an elec trical information bearing signal comprising an evacuated envelope, a source of electrons in the envelope, a target member at one end of the envelope, said target member comprising a mica element having a thickness which is no greater than 10 microns, a developing chamber disposed adjacent said target member, on the opposite side of the target member from the electron source, a plurality of electrostatically chargeable particles disposed in said developing chamber, said particles being substantially different triboelectrically from said mica element, means for controllably directing an electron beam from said source of electrons toward the target member to provide scanning of the target member, means disposed along the electron beam path for varying the beam intensity during scanning thereby to provide an electrostatic charge distribution pattern on the target surface, means for cascading the particles over the target member to cause said particles to adhere to the target member in correspondence to the electrostatic charge distribution pattern, means for directing a flood beam of electrons onto the portion of the target member receiving the electron beam to effect erasure of the electrostatic charges thereon, and a thin layer selected from the group consisting of substantially silicon dioxide composition and substantially silicon trioxide composition having a resistivity at room temperature of at least 10 ohm-cm. and a thickness which is no greater than 1200 Angstrom units, said thin layer being disposed on the side of the target member opposite 7 8 the electron source and adjacent the electrostatically 2,540,623 2/1951 Law. chargeable particles, said thin layer being closely related 2,587,830 3/1952 Freeman. triboelectrically with said particles and thereby prevent- 2,622,219 12/1952 Schagen. ing the charging of the surface adjacent the electrostati- 3,109,062 ,10/ 1963 Clauer et al. cally chargeable particles to prevent fogging of the target 5 3,401,294 9/ 1968 Cricchi et a1. 313-68 member.

2. The invention set forth in claim 1 above, wherein OTHER REFERENCES the triboelectric relationship between the thin layer and Dammanni Xerogfaphic Display IBM Techthe particles is such as to tend to charge the thin layer Ilical Disclosure Bulletin; V01. May 1962, page 47. positively relative to the particles in response to the 10 Engel? Electrographlc Developer for all Electfostatlc mutual agitation thereof, Storage Display Tube, IBM Technical Disclosure Bulletin, vol. 8, #12, May 1966, page 1737. References Cited UNITED STATES PATENTS 15 1,818,760 8/1931 Selenyi. -CLXR. 2,518,434 8/1950 Lubszynski. 178 7 87; 313 68; 340 173 ROBERT SEGAL, Primary Examiner 

